(1) Field of the Invention
The present invention pertains to a switch for an electric motor. More specifically, the present invention pertains to a switch having a resilient terminal arm that is slotted to reduce its biasing force exerted on an actuator of the switch.
(2) Description of the Related Art
Common capacitor start and split phase induction motors have a run winding and a start winding wrapped around poles of a stator of the motor. An example of this type of motor is disclosed in the U.S. Patent of Hildebrandt et al. U.S. Pat. No. 4,296,366. The start winding of the motor stator is energized during start up of the motor, or when the operating speed of the motor falls below a specified operating speed. Energizing the start winding of the stator creates a rotating magnetic field in the stator that applies a sufficient torque to the rotor of the motor to begin rotation of the rotor. However, once the rotor has begun its rotation and has reached a desired operating speed, it is able to follow the alternations of the magnetic field created by the run windings of the stator and energizing the start windings is no longer needed. Commonly, in motors of this type, the start winding is not intended for continuous use and may fail if not de-energized during normal run operation of the motor. Therefore, motors of this type are typically operated by a two position switch having an actuator that is moveable between first and second positions. In the first position of the actuator it closes a first set of electrical contacts that establishes a circuit through the start windings of the motor, and in the second position of the actuator it closes a second set of electrical contacts that establishes a circuit through the run windings of the motor while opening the first circuit of the start windings.
Two position switches of this type are typically moved between their two positions by a centrifugal actuator assembly mounted on the rotor shaft of the motor. FIGS. 1 and 2 show a two position switch 12 of the prior art and a centrifugal actuator assembly 14 mounted on the rotor shaft 16 of a motor (not shown). The centrifugal actuator assembly 14 rotates with the motor shaft and is responsive to the speed of rotation of the shaft for moving the switch actuator 18 from its first or start position to its second or run position in response to the rotation of the motor shaft attaining a predetermined operating speed. Some centrifugal actuator assemblies 14 include an annular collar 22 that is mounted on the rotor shaft for axially shifting movement between two positions, a start position of the collar on the shaft shown in FIG. 1, and a run position of the collar on the shaft shown in FIG. 2.
The co-assigned U.S. Patents of Hildebrandt et al. U.S. Pat. No. 4,296,366 and Lewis et al. U.S. Pat. No. 5,744,883 each disclose a two position motor switch that is acted on by a centrifugal actuator assembly. These patents are incorporated herein by reference. The same type of switch 12 is shown in FIG. 3 and includes a switch actuator 18 that resembles a bell crank. The switch actuator 18 is mounted by a pivot connection 24 to the housing 26 of the switch. The switch actuator has an exterior arm 28 that extends from the pivot connection 24 to the exterior of the switch housing and an interior arm 32 that extends from the pivot connection 24 to the interior of the switch housing. The exterior arm 28 is provided with a follower surface 34 on a distal end of the arm that engages with the annular collar 22 of the centrifugal actuator assembly mounted on the motor shaft. The interior arm 32 engages with two resilient terminal arms in the interior of the switch housing. The first or start terminal arm 36 is fixed to the switch housing at its proximal end and has an electrical contact 38 at its distal end. The start terminal arm contact 38 engages a first or start winding electrical contact 42 in the switch housing to close the circuit through the start winding of the motor. The second or run terminal arm 44 also is fixed to the switch housing 26 at its proximal end and has an electrical contact 46 at its distal end. The run terminal arm contact 46 engages a second or run winding electrical contact 48 in the switch housing to close the circuit through the run winding of the stator. Thus, the switch actuator 18, with its exterior arm 28 in sliding engagement with the cam surface of the centrifugal actuator collar 22, moves between two positions in response to the axial movement of the collar between its two positions on the rotor shaft 16. In the first position of the collar shown in FIG. 1, it positions the switch actuator 18 in its start position relative to the switch housing 26. This closes the circuit through the first, start winding terminal arm 36 of the switch, energizing the start winding of the motor. The start position of the actuator 18 is shown in solid lines in FIG. 3. When the collar moves to its second, run position on the rotor shaft shown in FIG. 2, the exterior arm 22 of the switch actuator slides over the exterior cam surface of the collar 22 allowing the exterior arm to move radially inwardly relative to the rotor shaft 16. This movement of the exterior arm is caused by the resiliency of both the first and second terminal arms. The first 13 and second 14 terminal arms exert a biasing force on the interior arm 32 of the switch actuator. The biasing force causes the interior arm to pivot about the pivot connection 24 in the switch housing. As the interior arm is moved, the resiliency of the first terminal arm moves its electrical contact 38 out of engagement with the start winding electrical contact 42 of the start circuit, opening the start circuit. Also as the interior arm is moved, the resiliency of the second terminal arm moves its electrical contact 46 into engagement with the electrical contact 48 of the run winding, establishing a circuit through the run winding of the stator. The run position of the actuator 18 is shown in dashed lines in FIG. 3.
As stated above, movement of the switch actuator 18 that causes the exterior arm 28 to move radially inwardly toward the rotor shaft 16 of the motor is caused by a biasing force exerted on the interior arm 32 of the switch actuator by both the first, start terminal arm 36 and the second, run terminal arm 44. The resiliency of the two terminal arms results in the arms functioning as leaf springs that each exert a biasing force on the interior arm of the switch actuator. As shown in FIG. 3, first 52 an second 54 abutments on the interior arm 32 of the switch actuator engage the respective first 36 and second 44 terminal arms when the actuator is in the start position, causing the terminal arms to bow upwardly between their opposite ends. The biasing force exerted by the terminal arms holds the exterior arm follower surface 34 in sliding engagement with the collar 22 of the centrifugal actuator. The biasing force also causes the exterior arm of the switch actuator to move radially inward toward the rotor shaft in response to the axial movement of the centrifugal actuator assembly to its run position on the shaft.
Although the two position switch functions well for its intended purpose, it has been observed that the biasing force exerted by the first 36 and second 44 terminal arms on the interior arm 32 of the switch actuator causes the follower surface 34 on the exterior arm of the switch actuator to engage in sliding contact with the collar 22 of the centrifugal actuator assembly with a force that increases the wear rate of the follower surface. In addition, the wear rate problem of the follower surface cannot be overcome by simply reducing the biasing force of the terminal arms because the biasing force of the start terminal arm 36 must be sufficient to break a weld that often forms between the contact 36 of the start terminal arm and the start winding electric contact 42 of the switch.
When the start terminal arm contact 38 engages with the start winding electrical contact 42 of the switch, the amount of current that passes through the engaging contacts causes the contacts to go through a molten stage producing a weld between the contacts. The resiliency of the start terminal arm 36 must exert a sufficiently large biasing force on the interior arm 32 of the switch actuator to assist in biasing the switch actuator from its start position to its run position, but it must also be sufficiently large to cause the contact 38 of the start terminal arm to break the weld with the start winding electrical contact 42 of the switch and separate from the contact, opening the start winding circuit as the switch actuator moves from its start position to its run position.